Rotary Dryer Research Articles

COMPARISON OF RESIDENCE TIME MODELS FOR CASCADING ROTARY DRYERS

ABSTRACT The predictions of the models of Matchett and Baker (1988) Saeman and Mitchell (1954) and Friedman and Marshall (1949) for the solids residence time in rotary dryers have been compared with both pilot-scale and industrial-scale data. A countercurrent pilot-scale dryer of 0.2m diameter and 2m long has been used with air velocities up to 1.5 ms−1 to measure the residence times of sorghum grain, The average discrepancy for the solids residence time between the predictions and the experiments that were carried out in the pilot-scale rotary dryer is — 10.4% Compared with the models of Friedman and Marshall (1949) and Saeman and Mitchell (1954) for the pilot-scale data obtained here, the Matchett and Baker model is more satisfactory for predicting the solids residence time in this pilot-scale dryer. It has also been found that the model of Matchett and Baker describes the industrial data of Saeman and Mitchell (1954) than the correlation of Friedman and Marshall (1949).

HYBRID NEURAL APPROACHES FOR MODELLING DRYING PROCESSES FOR PARTICULATE SOLIDS

ABSTRACT This work presents methods for synthesizing drying process models for particulate solids that combine prior knowledge with artificial neural networks. The inclusion of prior knowledge is investigated by developing two applications with the data from two indirect rotary steam dryers. The first application consisted in the modelling of the drying process of soya meal in a batch indirect rotary dryer, The external and internal mass transfer resistances were associated in the hidden layer of the network to linear and sigmoidal nodes, respectively. The second application consisted in the modelling of the drying process of soya meal in a continuos indirect rotary dryer. The model was constructed using the Semi-parametric Design Approach. The model predicts the evolution of solid moisture content and temperature as a function of the solid position in the dryer. The results show that the hybrid model performs better than the pure “ black box” neural network and default models. They also shows that prior knowledge enhances the extrapolation capabilities of a neural network model,

Reconciliation of Mass and Energy Data Measurements: Application to a Rotary Dryer

Data collected during the normal operation of a zinc concentrate rotary dryer is used to illustrate the problems associated with the calculation of unmeasured variables in the presence of measurement errors. The paper analyzes the propagation of random errors through mass and energy balance calculations and discusses the presence of systematic errors in measurements obtained during the operation of the dryer. The inaccuracy of temperature measurements in the presence of radiation is discussed, as well as the effect of air infiltration inside the dryer. The application of a reconciliation algorithm to the measured data slightly improves the quality of the estimation. © 1998 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.RésuméLes données recueillies durant l'opération d'un four rotatif de séchage de concentré de zinc sont utilisées pour illustrer les problèmes associés au calcul de variables non-mesurées en présence de'erreur de mesure. L'article analyse la propagation des erreurs de mesure à travers les calculs de bilan de masse et d'energie pour montrer l'imprécision de variables calculées comme les débits de gaz. L'influence d'erreurs systématiques de mesure de température par un thermocouple en présence de radiations est discutée et des résultats pratiques sont prsentés pour montrer la contribution des radiations émises par un mur de réfractaires ou par une flamme de combustion. On discute aussi de la précision de la mesure d'un débit d'air d'infiltration à l'interieur du four rotatif. Finalement les données sont utilisées pour evaluer lepotentiel d'un algorithme permettant de reconcilier des mesures de température, de débits d'air, de gaz et de carburant avec un modèle de conservation de l'énergie et de la matiére. © 1998 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.

Effect of Moisture Profile and Noncombustible Matter in Recycled Paper-Mill Sludge on Energy Recovery

Calorific values of solid sludge obtained from paper recycling mill rejects were estimated by first drying the sludge in a pilot scale rotary dryer and then determining the heal of combustion of these sludge samples. Sludge samples with varying moisture and inorganic contents were studied. Study revealed that a net thermal energy saving was possible by optimizing final moisture content of sludge before combustion. Sludge containing high amount of inorganic components needs to be dried to a final moisture content sufficient enough to ensure a net energy saving after drying and combustion. Thus, high inorganic containing sludge samples from a fine paper recycling mill generated significantly less energy compared to that of a deinking mill sludge which was more rich in fiber content. Although the presence of inorganic components, decomposable fillers in particular, and moisture in sludge negatively influenced the energy recovery, a net gain in the thermal energy was obtainable from all the three studied sludge compositions by optimizing the residual moisture content during drying of the wet sludge.

CALCULATING CAPACITY TRENDS IN ROTARY DRYERS

This paper provides a methodology developed for the calculation of the feed rate and of the exit air conditions in an adiabatic rotary dryer, which operates with granular, non-porous solids having only unbound surface moisture. Some aspects related to the algorithm are also discussed in greater detail, such as the behavior of the wet-bulb temperatures along the dryer and the selection of initial values for the iterative loops. The results have been compared with published data from commercial rotary dryers, and predictions compare within 10% of the available data. The methodology can be used to evaluate trends in the behavior of a rotary dryer where the operating parameters vary, and it is useful for the practical engineer, who has to manage several problems commonly encountered in the operation of a rotary dryer installed in a chemical plant.

Advanced Control of a Rotary Dryer

Two kinds of intelligent, hybrid control systems for a rotary dryer are presented. The main controlled variable is the output moisture of solids and the main manipulated variable is the input temperature of drying air which correlates to the fuel flow. The main disturbances of the process are the input moisture of solids and the feed flow. The one discussed control system includes a fuzzy logic controller (FLC) and a PI-controller and the other a neural network controller and a PI-controller. In both cases the intelligent controller determines the set point value to a PI controller. The control results have been examined both with simulations and with pilot plant experiments.

DETERMINATION OF RESIDENCE TIME OF GRAINS IN DRUM DRYER

On the basis of the theoretical analysts of the forces applied to grains and moving locus of grains in rotary drum dryer, residence time and contact heating time of grains in the dryer were calculated. The values of calculation conformed to the measured values. These results supplied reliable foundation in theory and practice for further studying and improving the dryer. They also provided designers with references to construct similar type of dryers.

Design of a semi-continuous rotary drier for vegetables

In this work the experimental performance of a drying module is presented as the first stage in the design of a modular rotary semi-batch drier for vegetables using hot air as a drying agent. The equipment will be made of a series of such modules, each one working as a batch stage where the material is dried up to a certain extent before passing to the next one, where the operation is repeated. Hence, the whole set-up operates in a semi-continuous mode, such that the first module receives the raw material load at regular time intervals, while the last one delivers the dry product. The operation has been modelled in order to calculate the instantaneous value of the drying rate as a function of the air conditions and the material water content. Integration of this equation with suitable heat and material balances provides a complete description of the air and solid properties at any time, which in turn allows the simulation and optimization of the complete set of modules. Experiments with onions, cut in small cubic pieces, have been carried out for constant values of other operating conditions, namely air velocity, particle size, speed of rotation and onion load, to determine the model parameters.

Dynamic simulation and control of direct rotary dryers

Adaptive and conventional control of a direct rotary dryer has been assessed through simulations. A dynamic model of the process was developed specifically for this purpose. The model is based on mass and energy balances and includes a drying kinetic derived from mass transfer principles. In the modelling process, ten discrete perfectly mixed elements were considered for the dryer with each containing four differential equations. A well tuned discrete PID and an adaptive algorithm were used to control the outlet solid humidity. Both controllers can regulate the controlled variable for different disturbances, although the adaptive controller is more effective.

Reconciliation of mass and energy data measurements: application to a rotary dryer

Data collected during the normal operation of a zinc concentrate rotary dryer is used to illustrate the problems associated with the calculation of unmeasured variables in the presence of measurement errors. The paper analyzes the propagation of random errors through mass and energy balance calculations and discusses the presence of systematic errors in measurements obtained during the operation of the dryer. The inaccuracy of temperature measurements in the presence of radiation is discussed, as well as the effect of air infiltration inside the dryer. The application of a reconciliation algorithm to the measured data slightly improves the quality of the estimation.

Dryer Effluent Monitoring in a Chemical Pilot Plant via Fiber-Optic Near-Infrared Spectroscopy

The monitoring of the effluent of a rotary dryer has been developed and implemented. The vapor stream between the dryer and the vacuum is monitored in real time by a process fiber-optic coupled near-infrared (NIR) spectrometer. A partial least-squares (PLS) calibration model was developed on the basis of solvents typically used in a chemical pilot plant and uploaded to an acousto-optic tunable filter NIR (AOTF-NIR). The AOTF-NIR is well suited to process monitoring as it electrically scans a crystal and hence has no moving parts. The AOTF-NIR continuously fits the PLS model to the currently collected spectrum. The returned values can be used to follow the drying process and determine when the material can be unloaded from the dryer. The effluent stream was monitored by placing a gas cell in-line with the vapor stream. The gas cell is fiber-optic coupled to a NIR instrument located 20 m away. The results indicate that the percent vapor in the effluent stream can be monitored in real time and thus be used to determine when the product is free of solvent.

MATHEMATICAL MODELLING OF INDIRECT CONTACT ROTARY DRTHRS

ABSTRACT Two different approaches were used to predict the solid moisture content and solid temperature profiles along a continuous indirect contact rotary dryer heated with steam tubes. One of these uses heat and mass balances applied to the solid phase in a differential element of dryer length. Here the heat flux is computed through an overall heat transfer coefficient assumed constant. The other model is based on a previous work that calculates the heat transfer coefficient as a function of the time the solid particles are in contact with the heating surface. The advantage in using this second model lies in the fact that the calculated heat transfer coefficient can take into account the effects of the operational conditions. If this coefficient has a strong dependence on these conditions, then it would be inappropriate to use a fixed value. Although both approaches can predict the solid moisture content and temperature profiles along the dryer differences were detected.

SIMULATION AND SCALE-UP OF PNEUMATIC CONVEYING AND CASCADING ROTARY DRYERS

ABSTRACT This paper presents a unified model for simulation of cocurrent and countercurrent dispersion-type dryers. The main industrial applications are to pneumatic conveying (flash) dryers and cascading (direct) rotary dryers. The basic model is a one-dimensional incremental (stepwise) simulation, which has been developed over a number of years Equations for particle motion, heat and mass transfer, heat and mass balances and local gas conditions are solved simultaneously over a small increment along the dryer. All workers have previously had considerable difficulty in obtaining a good fit between simulations and actual results from pilot-plants or large-scale industrial dryers. A new “fitting mode” calculation overcomes this by identifying the parameters which need to be adjusted, concentrating on those which cannot be measured accurately. Excellent agreement has been obtained between the model and experimental data by this method. The paper also presents revised formulations for particle motion and heat transfer in rotary dryers. The model has been incorporated into two computer programs for flash and rotary dryers respectively, and results from the former are shown for a case study.

Modeling Asd Optimization of Fluidized Bed and Rotary Dryers

ABSTRACT Fluidized bed and rotary dryers are two popular types of dryers utilized in almost every area of dryine. In this study, both dryers are analysed regarding their design and operational performance, so that they could be compared in a straightforward way. The mathematical models describing the convective drying process were evaluated for both types of dryers. Design procedures aiming at the determination of optimum sizing and operational characteristics for each type involved were carried out by appropriately optimizing the total annual cost of each structure for a given production capacity. Both dryer types were compared explicitely by evaluating optimum configurations for a wide range of production capacity values. Once the dryer sizing parameters are defined, its operational performance can be evaluated by comparing the optimum operational cost versus production capacity for predefined optimum designed structures. Rotary dryers turn out to be rather expensive compared to fluidized bed dryers, regarding design. On operational grounds, however, it is the other way around, due to the understandably favored heat transfer achieved in rotarv dryers. Characteristic examples covering a wide range of materials - from food products to inorganic minerals -are included in order to demonstrate the performance of each process as well as the effectiveness of the proposed approach.

Dynamics And Assessment of Some Control Strategies of a Simulated Industrial Rotary Dryer

ABSTRACT Rotary dryers are widely used for the continuous drying of minerals and chemicals on a large scale. Hot gases are passed parallel to the flowing solid to achieve the desired product moisture content. Because these dryers are energy intensive, it is mandatory to operate them as efficiently as possible to respond to economic pressures. Using a dynamic rotary dryer simulator for mineral concentrate, five control strategies are evaluated and compared. Two control strategies are based on PI controllers and the others use neural network models. Results clearly show that a feedforward action, in conjunction with a PI controller or incorporated within the structure of a neural network model, led to the best performances provided an accurate measurement of the feed moisture content is available.

Modelling and Dynamic Simulation of an Industrial Rotary Dryer

AbstractRotary dryers are widely used in chemical and mineral processes due to their flexibility and simplicity of operation. Nevertheless, the comprehension of the basic phenomena taking place in this type of dryer is still of great interest. In order to better understand the phenomenological behaviour of the process and to assess product moisture control strategies, a complete dynamic simulator of a mineral concentrate dryer has been developed The simulator is calibrated with industrial data and consists of a furnace model, a solids transportation model and a gas model, including the heat and mass transfer phenomena. The dryer simulator was shown to adequately represent the dynamic behaviour of an industrial rotary dryer.

Design and operation of convective industrial dryers

AbstractDesign and operational performance of convective industrial dryers are an important field of chemical engineering, which is still governed by empiricism. This article addresses the design vs. operation problem for three basic types of continuous convective industrial dryers: conveyor‐belt, fluidized bed, and rotary. Design procedures determined the optimal construction and operational characteristics in terms of total annual cost for each type involved and for a given production capacity through appropriate mathematical modeling. All dryer were compared by evaluating optimum configurations for a wide range of product characteristics and production capacity values. Once the dryer configuration was specified, its operational performance was evaluated by comparing the optimum operational cost vs. production capacity for predefined optimum designed structures. Rotary dryers were more expensive to design than fluidized bed dryers. Operationally, however, it is the other way around due to the favored heat transfer achieved in rotary dryers. Conveyor‐belt dryers lie somewhere between producing satisfactory results in terms of both design and operation. Case studies on foods and inorganics are included to demonstrate the performance of each process as well as the effectiveness of the proposed approach.

Hybrid-neural modeling for particulate solid drying processes

In this work, a general framework for modelling and simulation of particulate solid drying processes is presented, based on fundamental conservation laws associated with a neural network, for model uncertain parameter estimation. The modeling approach, which leads to a hybrid-neural model, is applied in order to describe the dynamic behavior of two important drying systems: a direct flow rotary dryer and a batch fluidized bed dryer. Both models are built using simple mass and energy balances, where heat and mass transfer parameters are estimated with neural networks. Model behavior was evaluated by comparing experimental and simulation data. It is concluded that the hybrid-neural modeling approach is better for adaptation and prediction than its black box type counterpart.

Drying of solids: Selection of some continuous operation dryer types

Design and study of operational performance of convective industrial dryers are a strong indicator in the last stages of the selection procedure. This problem is addressed in a straightforward way for the three most popular types; conveyor-belt, fluidized bed and rotary dryers. Design was carried out by appropriately optimizing the total annual cost of each configuration for a given production capacity. All dryer types were compared explicitely by evaluating optimal configurations for a wide range of production capacity values. Once the dryer configuration is specified, its optimal operational performance was evaluated by comparing the optimum operational cost versus production capacity for predefined optimal designed structures. Rotary dryers turned to be rather expensive compared to fluidized bed dryers, regarding design. On operational grounds, however, it is the other way around due to the understandably favored heat transfer achieved in rotary dryers. Conveyor-belt dryers, when utilized, lie somewhere in between producing satisfactory results both on design and on operational grounds. A characteristic example of food dehydration process is included, to demonstrate the effectiveness of the proposed approach.

Modeling of the Solids Transportation within an Industrial Rotary Dryer: A Simple Model

To better understand the underlying phenomena taking place in an industrial rotary dryer and to determine the optimum operating conditions, a simulator in which the solids transportation, the gas flow, and the heat and mass transfer are modeled is currently being developed. This paper describes the use of interactive perfect mixers in series to model the solids transportation within an industrial rotary dryer, on the basis of an experimental residence time distribution curve (RTD). Two simple models are proposed : a series of perfect well-mixed interacting tanks and a modified Cholette−Cloutier model. The first model is not able to account for the nonideal behavior of the solids transportation in the rotary dryer. To account for the characteristic extended tail of the RTD curves observed in industrial dryers, in a second model, the solid phase is divided between an active and a dead zone. This model, with 36 cells and 25% of the volume occupied by the dead zones, modeled very well the industrial RTD curv...